So what do Christians define abortion as?

Basically, if something has the natural capacity for human life, and is killed, then that is abortion. It doesn't matter what stage of life a fetus is in; as soon as the sperm and the egg connect there is life, and taking that life away causes death. Some argue that there is no difference between a bunch of human cells and a bunch of plant cells or animal cells- but there is an enormous difference. If you leave a bunch of animal cells alone, will they develop into a human? If you wait a year watching plant cells, will they eventually become a human baby? No. Contraception that prevents a sperm and an egg from connecting is true contraception. Contraception that kills a baby in its first stages of life is abortion.